The present invention is concerned with a membrane module, especially for performing separation of liquid mixtures according to the process of liquid permeation or pervaporation.
The state-of-art systems for the separation of substance mixtures have incorporated spiral-wound modules. Membranes suitable for use in the manufacture of such modules are arranged so that the inactive membrane faces are superposed, i.e. with the active faces thereof arranged outwardly, and are cemented together at the edges. Inserted into the resulting pockets are fleeces or cloths for draining the permeate. One or several of the pockets are secured to tubes provided with bores in the area of the cylindrical surface such that draining of the permeate can take place through the bores into the interior of the tube, there being no possibility of an outflow from the tube interior onto the active external surface (separating layer) of the membranes. Subsequently, the one or several membrane pockets, the draining discharge of which is in communication with the tube interior, are spirally wound about the tube, with interlayers of plastic fabric between the active membrane faces keeping open a liquid channel, and are sealed. The so obtained membrane modules thus obtained have two liquid channels separated by the membrane:
(a) the feed channels between the active membrane faces kept open in the axial direction by plastic fabric, and
(b) the permeate conduits spirally fed about the central tube, formed by the draining fleece or cloth and discharging the permeate passing through said membrane into the interior of the central tube.
These conventional spiral-wound modules are especially used for the separation of water from solutions, in accordance with the principle of reverse osmosis. They exhibit a high packing density (m.sup.2 membrane area per volume of module) and a high compressive strength; however, an essential disadvantage is involved therewith that has a substantial deterimental effect on special separating functions: the permeate passing through the membrane only through draining fleece via an extended path can be discharged into the central permeate collecting pipe, with the available flow cross-section being further reduced by the compressive load acting upon the active membrane faces. Moreover, substantial pressure losses occur between the interior of the permeate collecting pipe and the end of the draining fleece.
Different approaches have been tried to ease these inherent disadvantages of spiral-wound modules; however, practical experience has shown that when using such modules, satisfactory results can be obtained only if the volume ratio between intake and permeate does not substantially fall below a value of 10. While this is still acceptable in liquid-liquid separations in which no noteable change in the specific volume occurs upon passage through the membrane, a separation according to the pervaporation process cannot be performed economically with the state-of-art spiral-wound modules. In that process, the permeate vaporizes upon passage through the membrane and is discharged either by evacuation or by way of an inert gas stream. It is already by evaporation that the specific volume so substantially increases that the vapor volumes then under a reduced pressure can no longer be discharged through a draining fleece. As in the pervaporation process there is only a minor inter-membrane pressure difference, and pressure looses in the permeate channel cannot be accomodated. These disadvantages are avoided by the present invention.